The Claus process is widely used to produce sulfur from acid gas and other gases containing hydrogen sulfide In the modified Claus process feed gas containing hydrogen sulfide is partially combusted with air to form sulfur dioxide. The uncombusted hydrogen sulfide reacts with sulfur dioxide forming sulfur and water in a reaction furnace. The reaction stream is cooled and the sulfur is condensed and recovered. The reaction stream is then passed through one or more catalytic converters wherein additional sulfur is produced in these catalytic stages by the reaction of previously unreacted hydrogen sulfide with sulfur dioxide.
The amount of oxygen provided to the process is such as to be sufficient to combust about one-third of the incoming hydrogen sulfide as well as all of the other combustibles in the feed gas. This provides the proper stoichiometry between hydrogen sulfide and sulfur dioxide for the subsequent Claus reaction. Since air contains only about 21 percent oxygen, a significant amount of inert nitrogen is passed through the system. Furthermore, if the hydrogen sulfide concentration in the feed gas were to increase or if a higher gas processing rate is required, a greater amount of oxygen would be needed thus requiring an even greater amount of inert nitrogen to pass through the system. The increased flow of nitrogen increases the pressure drops in the system, reduces the residence time of the reactants in the reactors and increases the gas volume to be treated in the tail gas treating unit. Nitrogen is an undesirable diluent in the Claus process which, however, cannot be avoided if the feed gas is combusted with air.
Those skilled in the art have addressed the problem of this unproductive nitrogen throughput by employing oxygen or oxygen-enriched air as the oxidant and this technique can reduce the amount of nitrogen which passes though the system.
However, where the incoming feed gas contains a large concentration i.e. greater than about 50 percent hydrogen sulfide and other combustibles, the temperature of the combustion reaction when oxygen or oxygen-enriched air is the oxidant can exceed the temperature tolerance of the refractories in the combustion zone.
Those skilled in the art have addressed the problem of high combustion zone temperatures by recycling a portion of the downstream flow back to the combustion zone to dilute the combustion zone reactants and consequently reduce the combustion temperature. For example, U.S. Pat. No. 3,681,024-Hujsak teaches recycling a portion of the gas effluent from the last sulfur condenser to the combustion zone and U.S. Pat. No. 4,552,747 - Goar teaches recycling a portion of the gas effluent from the first sulfur condenser to the combustion zone.
However a problem with recirculation processes is that because nearly as much recirculated gas is required on a heat capacity basis as nitrogen was replaced by oxygen enrichment, any significant increase in the production rate can be achieved only by an increase in the total gas input rate to the combustion zone. However, such an increase in the gas input rate increases the pressure drop in and downstream of the reaction furnace up to the point where a fraction of the main gas stream is to be diverted for recirculation. Furthermore, the equipment associated with the metering, regulation and repressurization of the recirculated gas is vulnerable to breakdowns and may jeopardize the continuous operation of the plant.
Other temperature moderating additives which have been used or proposed for use in the combustion zone of the Claus reaction furnace include liquid water, liquid sulfur and liquid sulfur dioxide. Temperature moderation is achieved by the absorption of some of the heat released in the combustion zone by the temperature moderating additive. However the imposition of such an additive on the process stream increases the flow rate through the thermal stage, and unless the additive is removed from the reaction stream before the catalytic stages, it increases the pressure drop through the entire Claus plant. Thus, temperature moderation in the Claus reaction furnace by heat absorbing additives reimposes some of the inactive fluid load on the process stream, the elimination of which load was the reason for using oxygen or oxygen-enriched air in the first place.
Thus it would be desirable to have a Claus process wherein oxygen or oxygen-enriched air can be employed as the oxidant and wherein recirculated gas or other temperature moderating additives are not needed to maintain non-excessive temperature in the combustion zone.
It is therefore an object of this invention to provide an improved process for the recovery of sulfur by the combustion of feed gas with oxygen or oxygen-enriched air without the need for gas recirculation back to the combustion zone or introduction of exogeneous temperature moderating additives into the combustion zone.